Electric-furnace-balancing system



Sept. 30, 1924-.

R. D. EVANS ELECTRIC FURNACE BALANCING SYSTEM File y 13 1918 INVENTOR ATTORNEY WITNESSESI WW Patented Sept. 30, 1924.

UNITED STATES PATENT OFFICE.

ROBERT D. EVANS, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTING- HOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION OF PENN- SYLVANIA.

ELECTRIC-FURNACE-BALANCING SYSTEM.

Application filed May 13,

To all whom it may concern:

Be it known that I, Roenn'r D. EVANS, a citizen of the United States, and a resident of W'ilkinsburg, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Electric- Furnace-Balancing Systems, of which the following is a specification.

My invention relates to systems for imposing an unbalanced polyphase load upon a balanced polyphase supply system without disturbing the balance of the last named system, a

While the need for such a system is quite general, my invention relates more specifically to means for securing balanced power in a polyphase system from which an electric furnace, constituting an inherently unbalanced load, is fed.

In an are or resistance electric furnace,

in which two upper electrodes and one lower electrode are used, a tendency to load unbalancing obtains which, in turn, tends to unbalance the supply system. One of the objects of my invention is to provide a system which, after an initial manual setting, to be governed by the conditions obtaining in the furnace, automatically preserves the voltages and currents in the polyphase supply system in a balanced condition.

I have discovered that, by interposing between the supply system and the electric furnace a transforming aggregate the primary of which is Y-wound and the secondary of which is delta wound, said primary 35 and secondary being inductively interlinked in a particular manner, such balanced conditions are obtained.

For a more complete understanding of my invention, reference may be had to the accompanying drawings in. which- Fig. 1 is a diagrammatic showing of the transformer connections while Fig. 9. is a schematic illustration used to explain the voltage and current conditions in the primary and secondary windings of the transforming aggregate.

Referring more particularly to the drawings, there is shown a furnace 1, having upper electrodes 2 and 3 and a lower electrode 4 connected, by means of the leads 5, 6 and 7 to a secondary member 8 of a transforming aggregate 9, said secondary being delta wound and composed of the windings 10, 11 and 12. Inductively interlinked with the secondary 8 is a primary 13 havin its wind 1918. Serial No. 234,137.

ings 14, 15 and 16 Y-connected at a neutral point 17. It will be observed that the neutral point 17 is displaced from its correct position which, in this figure, is indicated at the junction of the dotted lines, these dotted lines, at the same time, indicating the correct phase position and magnitude for a symmetrical star winding. The primary windings 14, 15 and 16 are, in turn, connected to a polyphase supply source (not shown) by leads 18, 19 and 20. The leads 18, 19 and 20 are connected to the windings 14, 15 and 16 through the medium of adjustable taps, the voltages of the various phases being thereby adjustable in magnitude. It will be understood that the method of show ing the primary winding is merely schematic and is not indicative of the exact angular position obtaining between the phase voltages at all times.

Referring to Fig. 2, I will assume a set of conditions necessary for the drawing of balanced power from the source of supply when the secondary member of the transforming aggregate is connected to the electrodes and bottom contact of an electric furnace, the impedances of the paths between the electrodes 2 and 4 and between the electrodes 3 and 4, being equal and respectively less than the impedance of the path between the electrodes 2 and 3 and, therefore, resulting in the consumption of unbalanced power in the furnace. Inasmuch as both the current and voltage relations must be. considered, I find that the following mathematical deduction is far more feasible and understandable than any vector diagram which could be laid out:

Let-

/3 E=absolute value of line voltage.

Eza'bsolute value of voltage between each line and neutral.

a z'ratio of primary to secondary voltages on windings 14-11.

a zratio of primary to secondary voltages on windings 15 -12 and y,:admittance of the furnace load be tween the two upper electrodes.

y :admittance of the furnace load between a top and a bottom electrode.

nzneutral point of high voltage side.

k=ratio of E to The currents flowing in the secondaries of the transformers must have such values as to give the low-tension line currents and, moreover, they must be such that the sum of the primary currents is equal to zero.

These conditions give three equations from which the value of the secondary currents may be determined. These equations are:

I solve these equations by determinants, as follows:

1 C 0 0 C 1 a1 0 a2 2 5 1 4 The expressions for the low-tension currents may now be written by using the above results in connection with the equations previously given:

The primary currents are equal to the secondary currents multiplied by the ratio of transformation and are as follows:

: 2a, a,kEy, 20,11 (k 1 .5)Ey

The conditions for obtaining balanced power require that the voltages supplied to the primaries of the transformers be equal in magnitude and 120 displaced in phase, and that the resulting currents be equal in magnitude and 120 displaced. It is assumed that balanced supply voltages are applied to the transformer primaries and, in order to obtain balanced power, an-expression which will cause the currents to This operation gives the following'equation which must be satisfied in order to obtain balanced power:

1.7 32afa,kEy, 1.732a,a, (k 1.5)Ey

1.732afEy .866a,a, Ey,

From this equation the value of k may be obtained, as follows: i

In the above expression for balanced power we may now substitute the value of is, obtaining the following expression for the necessary condition for balanced power:

It will be observed that the left-hand member expresses the ratio of the impedances between the top electrodes, and between a top and the bottom electrode, in terms of the various ratios of transformation of the individual transformers. WVhen the above equations are satisfied, it is possible to obtain balanced power with a furnace having unequal impedances between the electrodes, by suitable displacement of the neutral on the high voltage side and by choosing the ratios of transformers so as to satisfy this condition in the impedances between the different electrodes.

It will be noted, in the foregoing analysis, that I have assumed the furnace itself to comprise a delta-connected load. This method admits of a somewhat more brief exposition of the proof, but it is to be understood that the same conditions hold if the furnace is represented as a star-connected load. In fact, in the latter instance, the impedances of the furnace leads themselves are considered in deriving the equation for maintaining balance and it follows there from that, assuming the cross section of the various leads to be the same, the length of the lead wires plays an important part in the amount of reactance which is needed to satisfy the above equation.

It will, therefore, be apparent that, by Y- connecting the windings of the primary and displacing the neutral point as desired, in ductively interlinking with this Y-wound primary a delta-wound secondary, and thereafter connecting said secondary, as has hereinbefore been explained, to the upper and lower electrodes of an electric arc furnace, I am able to impose, upon a balanced polyphase system, an unbalanced polyphase load, without disturbing the balance of the first named system. It will, moreover, be apparent that, after an adjustment of the transformer taps for the particular conditions existing, the system will cure the particular unbalance which that furnace load tends to impose upon the balanced polyphase system.

While I have described my balancing systcm in connection with an electric furnace, it is obvious that the same will be applicable wherever the same conditions of unbalance tend to appear.

Therefore, while I have shown but one embodiment of my invention, I desire only such limitations to be placed upon the same as the prior art may impose or as arespecifically set forth in the appended claims.

I claim as my invention:

1. In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase supply system without disturbance of the balance thereof, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming ag gregate being delta connected, the phase windings of said Y-connected' primary portion being so disposed and so connected that the electromotive forces therein may be in unsymmetrical relationship.

2. In combination with a polyphase sup ply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase supply system without disturbance of the balance thereof, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, the neutral point of said Y-connected primary portion being capable of displacement whereby distortion of the voltages in said Y-connected portion may take place.

3. In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase supply system without disturbance of the balance thereof, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, the neutral point of said Y-connected primary portion being adapted for displacement whereby the voltagesin the phase of said Y-connected portion may become distorted, and adjusting means for varying the magnitude of said distorted voltages.

4. In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and mino'r phases, upon said polyphase supply system, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, and said transforming aggregate having unequal voltage ratios, whereby said unbalanced polyphase load is imposed on said polyphase supply system without disturbing the balance thereof.

5. In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase sup ply system without disturbance of the bal ance thereof, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, said delta connected secondary portion consisting of major and minor phase windings.

6. In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase supply system without' disturbance of the balance thereof, said means comprising a polyphase transforming aggregate inductively interlinking said load and said system, the primary portion of said transforming ag gregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, said delta-connected secondary portion consisting of major and minor-phase windings, said major-phase winding being connected to that portion of said unbalanced polyphase load which has the greater impedance.

7 In combination with a polyphase supply system, means for imposing an unbalanced polyphase load, consisting of major and minor phases, upon said polyphase supply system without disturbance of the balance thereof, said means comprisin a pol phase transforming aggregate in uctivel y interlinking said load and said system, the

primary portion of said transforming aggregate being Y-connected and the secondary portion of said transforming aggregate being delta connected, said delta connected secondary portion consisting of ma.- jor and minor hase windings, said majorphase winding being connected to that portion of said unbalanced polyphase load which has the greater impedance, and said minor-phase windings being connected to those portions of said unbalanced polyphase load which have the lesser impedances.

8. The combination with an electric furnace having upper and lower electrodes, and a polyphase supply system therefor, of a transformer system comprising a-Y-connected primary and a delta-connected secondary interposed between said electric furnace and said polyphase supply system, said trans former system operating to cure the unbalanced conditions introduced into said polyphase supply system by said electric furnace.

9. The combination with an electric furnace, and a polyphase supply system therefor, said furnace operating in such manner that an unbalanced load is imposed on said supply system, of a transforming aggregate interposed between said furnace and said supply system, the primary and secondary phases of said transforming aggregate being so connected and so inductively interlinked as to cure the unbalanced conditions imposed on said polyphase supply source.

10. The combination with an electric furnace having upper and lower electrodes, and a polyphase supply system therefor, of a transforming aggregate comprising a Y- wound primary and a delta-wound secondaryinterposed between said furnace and said polyphase supply system, the neutral point of said Y-wound primary being so arranged that the electromotive forces in said primary winding may become distorted and thereby cure the unbalanced conditions introduced into said polyphase supply system by said furnace. i

' 11. In connection with are electrode furnaces, the improved method of distributing three-phase currents so that the supply phases tend to have balanced loads, consistlng in the arrangement of three transformer secondary windings grouped in delta fashion, the terminal junctions of which are connected respectively to two up er electrodes or groups of electrodes and a third electrode or conductive hearth, and three transformer primary windings grouped in star fashion, the ratio of transformation between the various transformer windings being arranged so that the three primary phases of electrical supply are equally loaded for power and power factor when the secondary system comprised by the furnace is symmetrically loaded, that is when the two upper electrodes are each conveying equal power, substantially as herein described.

12. In connection with the arrangement claimed in claim 19, the specific voltage relationship between the voltages impressed on the separate transformer primary and secondary windings to create a balanced system when the hearth of the furnace has 10 no electrical resistance and for all'intermeplled by an equivalent phase of secondary 15 current supply, as herein set forth.

In testimony whereof, I have hereunto subscribed my name this 30th day of April,

ROBERT D. EVANS. 

